The present invention is directed to a method for supplying power to an element of a source of radiation. In particular, the present invention is directed to a method for supplying power to a heating filament of a cathode of an X-ray tube. The invention can be used in medicine, especially in vascular type applications. The present invention is directed to the quality of images produced with X-ray tubes. The present invention also relates to the X-ray tube itself.
For the acquisition of a radiology image, an object, such as a body of a patient, is subjected to irradiation by X-rays, which go through the object and are partially attenuated in the object, the remaining irradiation being sensed by a detector, i.e., a film or an electronic detector. Instead of solid X-ray sources, electron tubes capable of producing X-rays are used as the source of radiation. Electron tubes are more flexible in their use. Electron tubes can be used to dictate the hardness of the X-rays produced (related to their energy and hence to the frequency of the photon radiation) and to the delivery rate of the X-rays produced.
The delivery rate of the X-rays is chosen as a function of the results of the measurements that are developed by means of an integration of the energy collected at the detector. Furthermore, to simplify the description, the larger the object the greater is the delivery rate needed if a significant part of the X-rays is to reach the detector. Since the detector has an energy-related dynamic range for developing results, the mean quantity of energy received by the detector, per surface element, should be located in the middle of this dynamic range (or at an expected value) so that the image contrast is distributed as efficiently as possible. If the accumulated energy is excessively strong, the detector is saturated and there is a loss of contrast for the transparent parts of the object. If, on the contrary, the energy received is too weak, the detector is under-exposed, and there is a loss of contrast for the thickest parts of the object.
The hardness of the X-rays is chiefly controlled by the high voltage between an anode and a cathode of the tube, while the delivery rate of the X-rays depends chiefly on the heating current of the anode. For the hardness the electrons liberated from the cathode strike the anode at speeds that are especially high as the high voltage is elevated. This striking of the anode leads to the production of X-rays of high energy value. And the same time, the number of the electrons that can be liberated from the cathode to be projected on to the anode depends especially on the state of excitation of the cathode which itself depends on its thermal state. Ultimately, the flow rate of the tube current, which is directly related to the X-ray delivery rate, is thus linked to the temperature of the tube.
The acquisition of a radiography image and, more generally a radiological examination therefore requires that, once the object, such as a patient, has been placed in an intermediate position between the tube and the detector, the tube should be made to send out irradiation during the exposure. The duration of the exposure is another multiplier factor of the accumulation of the energy sensed by the detector. For reasons of excessively fast wear and tear of the cathode through the spontaneous liberation of electrons, there are known ways of heating the cathode only when it has to make an emission. In practice, the cathode can be kept at a temperature far below the high temperature (around 4000° K.) that is its service temperature.
The pulsed operation to which the tube is subjected then runs up against a difficulty related to the time constant of thermal heating of the cathode. This difficulty delays the setting of the tube at its temperature. A cathode at excessively low temperature would send out an excessively weak tube current and, for a given duration of irradiation, the cumulated energy of the X-rays emitted would be different from the expected cumulated energy.
In order to overcome this problem, there is a known way of preheating the cathode, prior to the emission impulse, so that it reaches its service temperature. This preheating is however fairly slow and takes about four to five seconds. Such slowness is of course unacceptable in certain fields, especially in the vascular field where a contrast product is sent into the patient's blood at the same time as a radiographic exposure is taken of the arterial and venous distribution systems. This contrast agent spreads in the blood, in the form of a wave, imposed by the heartbeat. In other words, the improved contrast is visible only transiently, for a period close to one second and at a date that is a random date and related to the injection date and, in any case, having little compatibility with the waiting period of four or five seconds.
To overcome this problem, there are known ways of passing from the value of an electrical holding current (enabling the holding of the cathode heating) to a service current (corresponding to an expected X-ray delivery rate) by means of short-duration pulse imposing a boost current value on the heating current. For one and the same thermal time constant, the evolution in temperature of the cathode is then considerably quicker. After a calibrated duration of this boost current, generally equal to 400 milliseconds, the heating current of the cathode is imposed on a service value. This service value is in between the value of the holding current and the value of the boost current.
Generally, at the end of a subsequent stabilizing period that, in one example, is itself also equal to 400 milliseconds, the irradiation proper can be carried out. This irradiation, depending on the tube technologies used, may be prompted either by the switching of the high voltage between anode and cathode or by the switching of a voltage of the control grid interposed between the cathode and the anode. Such an approach gives good results, in any case better results than those obtained when the temporary boost current is not applied.
However, modem requirements as regards the control of the delivery rate are far greater. In particular, the mean delivery rate of the tube during the pulse should be contained within a window of ±10% about an expected mean value. It has been realized that, despite the boost current, major disparities occur and that the tube current cannot be controlled with the desired precision.